Fix no-op transaction error on paste.create Conduit API method

Summary:
Fixes T9735. I changed how the TYPE_LANGUAGE transction works a little but that accidentally tripped an error condition in `paste.create`.

  - Don't bail on no-effect transactions to `paste.create` (like not setting a language).
  - When a transaction type has no tailored UI message, make it easier to figure out which transaction is problematic.

Test Plan: Ran `arc paste ...` locally. Got an error before the patch, clean paste creation afterward.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9735

Differential Revision: https://secure.phabricator.com/D14440

src/applications/paste/conduit/PasteCreateConduitAPIMethod.php

@@ -60,6 +60,7 @@ final class PasteCreateConduitAPIMethod extends PasteConduitAPIMethod {
 
     $editor = id(new PhabricatorPasteEditor())
       ->setActor($viewer)
+      ->setContinueOnNoEffect(true)
       ->setContentSourceFromConduitRequest($request);
 
     $xactions = $editor->applyTransactions($paste, $xactions);

src/applications/transactions/storage/PhabricatorApplicationTransaction.php

@@ -626,7 +626,9 @@ abstract class PhabricatorApplicationTransaction
         return pht('Edges already exist; transaction has no effect.');
     }
 
-    return pht('Transaction has no effect.');
+    return pht(
+      'Transaction (of type "%s") has no effect.',
+      $this->getTransactionType());
   }
 
   public function getTitle() {